1. Field of the Invention
The present invention relates to a method of fabricating a printed wiring board, particularly to a method of fabricating a printed wiring board having a slot for a bonding wire for connecting with a semiconductor chip at its central portion.
2. Description of the Related Art
As an example of a conventional method of fabrcating a printed wiring board, conventionally, a printed wiring board is fabricated by using a copper-clad glass epoxy resin substrate formed of a glass epoxy resin having a copper coating on a surface thereof. Such a copper-clad glass epoxy resin substrate is provided by a method in which a copper foil previously coated with an adhesive agent on a bonding face thereof is pasted on a glass epoxy resin board produced by impregnating epoxy resin in glass fabrics, or by a method in which a glass epoxy resin prepreg and a copper foil are subjected to thermal compression bonding.
A copper foil used as a copper coating layer formed on this kind of a copperlad glass epoxy resin substrate is an electrolytic copper foil having a thickness of about 9 xcexcm to 35 xcexcm.
However, since printed wiring boards are used widely in apparatus for domestic use such as television sets, cameras and in various kinds of industrial apparatus such as computers, high density wiring is needed. As a result, thin wall formation of a copper coating layer has been requested in order to accurately form fine patterned circuits. Therefore, according to the copperclad glass epoxy resin substrate, in accordance with the thin wall formation of the copper coating layer, a method of forming a copper coating by electroless plating has been developed. Further, as a method of fabricating a printed wiring board by using such a copper-clad glass epoxy resin substrate, a semi additive process or a subtractive process has generally been used.
The semi additive process is a process in which a conductor circuit is formed on the copper coating layer on the copper-clad glass epoxy resin substrate by plating, and specifically a metal coating formed on the copperciad glass epoxy resin substrate provides a first metal layer and a plating resist layer is formed thereon according to a predetermined pattern.
In this case, a thickness of the plating resist layer is made to be equal to or larger than a thickness of a circuit formed on the first metal layer by a plating process. Successively, a plate coating (second metal layer) is formed on the metal coating (first metal layer) exposed among intermediate portions of the pattern of the plating resist layer by an electric plating process. Thereafter, the plating resist layer is exfoliated, a face of the metal coating (fust metal layer) is exposed and the exposed metal coating (first metal layer) is removed by etching with the electrically-plated coating (second metal layer) as a mask to thereby form the conductor circuit.
The subtractive process is a process in which a conductor circuit is formed by removing a copper coating layer from the copper-clad glass epoxy resin substrate, and specifically an etching resist layer is provided on the copper coating formed on the copper-clad glass epoxy resin substrate and is formed in a pattern indicating a region necessary for the circuit. Successively, a conductor circuit pattern is formed by dissolving to remove the exposed metal coating region and thereafter, the resist layer is exfoliated to thereby fabricate a printed wiring board. According to the printed wiring board provided by these processes, in comparison with a conventional printed wiring board prepared by using the copper-clad glass epoxy resin substrate fabricated by the copper foil as its material, wirings having a high density can be provided.
Further, the following process is carried out thereafter on a conductor circuit portion of the printed wiring board prepared as described above.
That is, after roughening the conductor circuit portion in order to promote adhesion thereof with a solder resist, provided later, the solder resist is coated or laminated, a bump pad portion is expose at a central portion by exposing and developing the solder resist layer, the resist layer remaining at a surrounding portion is thermally harlened, and thereafter the bump pad portion is electrically plated
Further, the printed wiring board formed as described above is provided with a shape as shown in FIG. 5. The printed wiring board is constructed such that bump pads 14 for connecting to electrode pads 13 of a semiconductor chip 12 are aligned in arrays at peripheral portions of a printed wiring board 11 and the electrode pads 13 of the semiconductor chip 12 are connected to the bump pads 14 by bonding wires 15, as illustrated in FIG. 6. Further, there is formed a semiconductor package of PBGA molding the printed wring board 11and the semiconductor chips 12 connected as described above by a resin 16 or TSGA integrating tape substrate on which the wiring is provided and the semiconductor chips 12 by a cover plate, as required.
However, for conventional printed wiring boards arranged with the bump pads aligned in arrays at the peripheral portions of the printed wiring substrate, the electrode pads of the semiconductor chip are connected to the bump pads at the peripheral portions of the printed wiring substrate by bonding wires. Accordingly, a wiring length of the bonding wires must be relatively long and thus the conventional printed wiring board cannot be used for a conductor circuit having a higher density or high frequency, as now in demand. Hence, in order to shorten the wiring length, there has been proposed a printed wiring board formed by perforating a slot using a router bit, the slot being connected to a semiconductor chip by bonding wires at a central portion of a printed wiring substrate and meanwhile, arranging bump pads at vicinities of the slot and including bonding wires for connecting to electrode pads of semiconductor chips to the slot.
According to the printed wiring board of this type, the conductor circuit is formed by the semi additive process or the subtractive process by using the copper-clad glass epoxy resin substrate. Successively, a surface treatment is carried out on the conductor circuit by electric plating, and thereafter a slot is provided at the central portion of the substrate by the router bit. The pattern of the wiring portion of the conductor circuit on the printed wiring substrate is formed orthogonally relative to a moving direction of the router bit. The wiring pattern of the conductor circuit is simultaneously cut when the slot is perforated.
However, when the slot is perforated by the router bit, cut burr is produced at cut surfaces of the wiring portion formed orthogonally to a moving direction of the router bit. When the burr is large such that it will project towards an area of slot, and when the pattern is pulled by the router bit and causes exfoliation of the wiring during cutting the wiring pattern, the printed wiring board becomes unusable, and when the burr is small, the burr must be removed by vacuum suction or the like. Therefore, time and labor are required, the product yield is reduced and the production efficiency is lowered. Further, when the burr is not completely removed, short circuiting between the wires can occur.
It is an object of the invention to provide a method of fabricating a printed wiring board so as to prevent cut burr from being produced at cut surfaces of the wiring pattern during cutting the wiring pattern, and simultaneously preventing peel off of the wiring, whereby the product yield is promoted and the operational efficiency is improved.
In order to achieve the above-described object, according to an aspect of the invention, the present invention provides a method of fabricating a printed wiring board. The method includes the steps of forming a conductor circuit by patterning a metal coating formed on a surface of an insulating substrate, successively subjecting the conductor circuit to a surface treatment by electric plating, and thereafter perforating a slot for bonding wires for connecting to a semiconductor chip at a central portion of the substrate by a router bit. According to the method described above, a pattern of a wiring portion constructed of the conductor circuit at a location for perforating the slot is inclined to at an acute angle relative to a face orthogonal to a moving direction of the router bit, preferably, an acute angle of approximately 15xc2x0 or more. Moreover, for the method described above, a width of the wiring portion for the electric plating is approximately 90 xcexcm or more, preferably, 120 xcexcm or more.